In current power amplifier (PA) designs, linearity and power efficiency are important requirements. In fact, the design space of a PA consists of several parameters affecting the linearity and efficiency requirements, such as compression point, output power, available gain or accuracy, which can be expressed by an error vector magnitude (EVM). It is very hard to find an optimal designed amplifier because it exists a trade-off between some of the parameters, so that not all of them can be optimized at the same time. For instance, efficiency and linearity requirements are two contradictory requirements. In principle, increased linearity for high power levels results in less power efficiency and, on the other hand, increased efficiency for low power levels results in poor linearity. The above problem becomes even more demanding for wireless communication systems having amplitude and phase modulation, e.g., quadrature amplitude modulation (QAM). Especially for wireless communication systems using orthogonal frequency division multiplex (OFDM) as modulation scheme, the above problem gets even worse due to the fact that OFDM signals possess high peak to average ratios (e.g. 10 dB) which imposes another parameter on the PA design. Such high peak to average ratio requires class A and AB driving schemes for the PAs of these OFDM communication systems. However, using class A and AB leads to a significant reduction of efficiency of the PAs.
As an example, Wireless Local Area Network (WLAN) PAs need to provide power levels in the order of 19 to 21 dBm at the output and have to meet EVM requirements with a power aided efficiency (PAE) of 20 percent or higher.
In order to solve the above efficiency requirements, polar modulation technology has been developed to facilitate system design. A polar modulator can independently process a carrier's amplitude and phase signals, typically working together with a non-linear power amplifier operating in switched mode. The elimination of the linear operation requirement enables power amplifier efficiency to be maximized for each modulation standard. Under a polar modulation scheme, multimode operation may be achieved by digital switching. Phase information is used to tune a voltage-controlled oscillator (VCO) driving the PA, while amplitude information modulates the PA according to the required standard. Consequently, when using polar modulation, the complex baseband signal is split up into phase and amplitude components which are combined at the output stage.
US 2004/0212445 A1 discloses a method and apparatus for polar modulation where in-phase (I) component and quadrature-phase (Q) component are digitally processed to produce digital envelope or magnitude and phase signals. In the phase signal path, the phase signal is converted to a baseband frequency signal by taking the derivative of the phase with a digital derivative circuit. Furthermore, a digital time delay circuit is used or controlled to synchronize the phase path with the magnitude path where the magnitude signal is differentiated and converted to an analog signal used to drive the PA. The digital baseband frequency signal is then supplied to a fractional divider circuit of a phase locked loop (PLL) arrangement to generate a sinusoidal wave. The above additional digital processing in the magnitude and phase paths leads to improved signal quality in the described polar modulation system.
However, conversion of Cartesian signals, such as in-phase (I) and quadrature-phase (Q) signals, to polar signals usually requires a non-linear operation. Due to that, the bandwidth of the envelope and phase component of the polar signal is significantly enlarged up to 5 or 8 times the bandwidth of the Cartesian signals. In the digital domain this would lead to additional aliasing, which increases EVM.